Many field-effect transistors (FETs), particularly high electron mobility transistors (HEMTs), have been reported as compound semiconductor devices. In particular, an AlGaN/GaN HEMT, including a GaN electron transit layer and an AlGaN electron supply layer, has been receiving attention lately. In an AlGaN/GaN HEMT, distortion occurs in the AlGaN layer due to the difference in lattice constant between GaN and AlGaN. This causes piezoelectric polarization, and a high-concentration two-dimensional electron gas (2DEG) is produced by the piezoelectric polarization and AlGaN spontaneous polarization.
In HEMTs, if holes generated by impact ionization in a high-electric field are accumulated in the electron transit layer, some problems occur. For example, the withstand voltage of the device may be reduced, or the Kink effect may occur, thereby varying the threshold voltage of the gate or vary the drain current. It is known that these problems can be effectively solved by extracting (discharging) the holes generated by impact ionization from the electron transit layer.
For example, in Japanese Unexamined Patent Application Publication Nos. 2001-284576, 2007-329205, 2006-173582, and 2001-168111, an electrode is provided on the surface or rear side of the electron transit layer to extract the holes from the electron transit layer.
The holes generated by impact ionization can easily transfer to the rear side of the electron transit layer due to the inclination of the vertical valence band in the device. Accordingly, it is expected that the holes can be effectively extracted by providing a hole extraction electrode on the rear side of the electron transit layer.
However, in the structure in which a hole extraction electrode is disposed directly on the rear surface of the electron transit layer, it is difficult to reliably extract holes. A hole extraction electrode may be provided on the rear surface of a p-type GaN layer formed under the electron transit layer. In this instance, however, the entire rear surface of the p-type GaN layer is covered with an electrically conductive layer. Consequently, a capacitance is generated between the upper side and the lower side of the HEMT and the high-frequency performance of the HEMT is degraded. Also, since a high electric field is applied between the drain electrode and the hole extraction electrode, the withstand voltage can be reduced.